Traveling back in time could be possible | Hacker News Reader

Traveling back in time could be possible

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56 pointsjurjenh51 comments

Note that under our current understanding of General Relativity, wormholes will rapidly collapse under their own gravity so quickly that not even a single photon would be able to traverse it because it closes off.

There's a theoretical way to stabilize it: put 'exotic matter' in the middle. Unlike all other matter, exotic matter has a negative energy density. This is the same sort of matter that would be needed to create an Alcubierre warp drive.

Unfortunately, we don't have any idea how to go about creating exotic matter. The closest we've come is the Casimir effect, when a region between two very close conductive plates will in some ways act as if there were negative energy.

[1] http://www.bibliotecapleyades.net/ciencia/negativeenergy/neg...

This is such a fascinating comment - can you please give me an ELI5 out-of-the-loop comment to help me understand

Just a quibble, you are referring here to a form of matter with a specific exotic property, which is not a property necessarily held by all forms of exotic matter.

TLDR: theoretically possible with hypothetical extrapolations of existing physics, but incredibly unlikely that the needed extrapolations are both possible and viable for use by humans

> Unfortunately, we don't have any idea how to go about creating exotic matter

A priori, I don't see any reason to believe that creating it should be possible. It's never been observed, and I'm assuming there aren't any models which predict any methods by which it might come into existence. That's as good as saying it's made up to me.

I broke my rule of not reading medium articles, but how exactly does this prevent the grandfather paradox? Once you're back in time you can still stop yourself in the future from entering the worm hole...

You'd need to physically catch up to yourself (your parents, in the case) which is impossible given they are traveling at the speed of light, in chase of the other end of the wormhole. At least that's what I could gather.

Wouldn't another wormhole going the other way allow you to get around this?

Why not send the wormhole on a circular journey? Accelerate out, decelerate, accelerate back, decelerate. Enjoy two relativistic trips, and end up parked side-by-side.

This is the mechanism Stephen Baxter used in his Xeelee sequence novels.

Where's the paradox? Given that a person has already appeared out of nowhere - matter/energy has been created - I'd say that having someone with identical genetic make-up to them running around is minor stuff.

I exist in 2017. I go to 1978. I kill my other form before 2017 is reached. Therefore I cannot have traveled to 1978. What has occurred at each particualy moment is no longer consistent in a way that is clearly comprehensible by humans.

In 1978 the other you would be 1 year into their light-speed trip away from Earth. How can you kill them?

As mentioned in other comments, another wormhole or a more direct path to cut him off.

Actually, of interest Wikipedia seems to say that you wouldn't have to leave Earth at all (just bring the other half of the wormhole back), so I think you're right that it doesn't solve the grandfather paradox: https://en.wikipedia.org/wiki/Wormhole#Time_travel

Cool, seems we've reached the bottom of this one!

You have it backward.

The only reason you managed to live into 2017 is because all the assassinations on your grandparents failed.

The only thing that will happen if you try to go into 1978 is that you will discover why you failed.

Hahaha sure but this scifi scenario is orthogonal to the wormhole time travel mechanism.

Essentially you end up with bi-directional causality.

I think the last part is key: comprehensible to humans. I see the paradox but it’s a logical one not necessarily a physical one. I’m not even sure what time travel really means. Do you look at a river and think to capture it exactly as it was an hour ago? Would it really be the old river then? If you did capture it and add a few extra drops then set it in motion would there really be a problem? I’d posit that the past does not exist except as a mental construct to begin with. A “present moment” doesn’t even exist, how could you have a past moment to return to?

It's either perfectly consistent if you go minute by minute, or consistency goes out the window without any murder.

1978: Some matter and energy materializes. It appears to be a human being. Where did it come from? It's impossible to tell; it has no past as far as we're concerned.

1978: Oh my god, the mystery human has just killed another human. They look kind of alike, I guess.

1978: The mystery human is now in prison; its lack of citizenship or identity ("yeah, born in 1952 my ass, you're clearly at least 50 years old") does not prevent its incarceration.

2017: Nothing of note happens. Mystery human would be eligible for parole, if he hadn't died behind bars in 1996.

But maybe when you go back you instantaneously become you? That would close the loop. The future you had essentially ceased to exist anyway so there's no reason to go back.

By your logic any machine that kills you is a time-machine. "See, now you exist in the past."

I'm so happy with this comment right now, we've discovered time travel on THN!

If we're going to toss in "magically de-age by about 30 years", we might as well start talking about my other two genie wishes.

> matter/energy has been created

This is where the “exotic matter’s” negative energy comes in.

One proposal is the Novikov self-consistency principle.

Imagine a billiard table with a wormhole, which curves and goes three seconds backwards in time. It's set up so you can roll a ball into the wormhole, and it will emerge three seconds earlier and knock itself off the path, so it doesn't enter the wormhole. Paradox!

Except actually it emerges with a slightly altered path, and strikes itself only a glancing blow.

And why did it emerge with an altered path? Because it was struck a glancing blow.

https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

So, circular causality, basically?

Why have you sworn off medium articles? Is the quality too consistently low, or has medium as a company done something to annoy you?

I consider their quality to be consistently low. This one is basically a weak summary of a Wikipedia article or two along with some overzealous assumptions.

Either that, or article writing AI is getting surprisingly good.

I wouldn't be surprised. A lot of the articles have a similar tone.

Funny for me to read this, because I’d sworn off Medium articles too for the same reason, yet never heard other people talking about it.

I wouldn’t say it’s quality overall - many are well-written - rather that virtually none bring anything new to the table, have any insight into their subject. Something about the platform draws people whose goal is to write, not who have something to say.

For my part I think there is a kind of sophomoric "look at me" quality to most Medium articles that makes them mostly about promoting the author rather than pursuing any type of truth. This is why I stopped reading them, I get a twinge of annoyance every time I see medium, because I feel that I am about to read someone's sales pitch as to why they are the smartest, rather than read a genuine pursuit of truth.

The idea is that you can't do that because the conditions necessary for you to be at the entrance of the time travel working prevent you from being at the exit of the wormhole at the same time.

To get to the entrance, you must leave the Earth (with the entrance) at time X. After you've left, you can enter the wormhole and emerge from the Earth side at time X+e.

You leave Earth in 2020. You leave one end of the wormhole here, and take the other with you on a near-light-speed journey 40 light years away. One year goes by for you due to relativity, you pick up a burger from alien McDonald's, then step into your wormhole- which exits in 2021, not 2040.

Edit: no wait, I can't be understanding this right. You could just journey back home with your wormhole and give it to someone else.

I like to think of it as a feedback loop stabilizing. I think it's the model used in Harry Potter. We won't be able to observe what happens during the stabilization. Paradoxes become impossoble then.

It's not clear what all the rules are and how this prevents the grandfather paradox.

What ever mechanism that prevents the paradoxical violations also makes this not very exciting (as far as time travel goes): you have to spend the "40 years" between you visiting one end of the wormhole and you entering the other at near-luminal velocity (hence you would only perceive it as 1 anyway). It's not so much "time travel" as an instant return ticket (where only your personal real time is spent, assuming your ship is exactly as fast as the one that carried the wormhole).

Which does actually have an interesting effect if you make them in pairs. It makes books like The Commonwealth Saga technically almost viable (e.g. where they build networks of portals and then just run trains through them non-stop to travel between worlds). Except that instead of using portals to explore new worlds, they would have to send a probe to those planets with a wormhole on board, at near the speed of light. And trips might cost you a bit of relative time to make a trip and return (e.g. the difference between the speed of light and the speed the probe delivered the wormhole from both portals added together would be how far in the relative future a trip would send you).

This all of course requires exotic matter which probably doesn't exist; which is why this whole thing probably doesn't work in the first place (e.g. why stable wormholes probably don't exist). Still might make it's own excellent sci-fi book if someone ran with the constrains of this, and it would technically be harder sci-fi than most.

I don’t see how time travel doesn’t violate the law of conservation of energy or conservation of mass. That seems pretty fundamental to me, so it would require converting an extremely large amount of energy into the mass that is going back in time.

What if you can only send and receive information?

Theoretically yes, but what they didn't tell you is that all those particles travelling through will completely lose all their information and order how they belong together. So please dear sci-fi authors stop with these ridiculous plots.

If I read that correctly, you are never able to travel back further than the creation of the wormhole.

Sounds like the movie Primer

The real question I have is how would you ever stabilize something as powerful as a black hole.

universe> ssh god@blackhole-42

universe@backhole-42> configure

universe@backhole-42> set stable MAX_VALUE

universe@backhole-42> commit

..changes successfully committed to blackhole-42

universe@backhole-42> exit

universe>

It seems we have laws of physics covering everything from Newtonian mechanics, relativity and quantum mechanics... Is there any law stating time travel must go one direction? Has it ever been proposed?

Second law of thermodynamics https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

>The Second Law of Thermodynamics states that the state of entropy of the entire universe, as an isolated system, will always increase over time

My understanding (which is very small) is that the multiverse theory takes care of not violating this law, since it’s not an isolated system.

And....there is no evidence at all that there is another Universe other than ours. Once you have proven the existence of a multiverse we can then take theories that require it seriously. Until then it is pure fantasy.

> If this negative mass/energy matter exists

Once

Travelling backwards in time is not possible, it's obviously not possible, and it's a wonder that the idea gets given such credence in educated circles, apart from it being a nice thought and a diverting intellectual exercise.

Actually, time travel is real. My name is John Titor, from the year 2036. I'm looking for an old IBM 5100, does anyone here know where I can find one? The future of the world depends on it!

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